This invention relates to control systems for video tape recorders and, more specifically, to control systems which, during reproduction, control the across track positions of reproducing magnetic heads, that is, the positions of the heads considered in the direction transverse to the tracks being scanned.
In a helical scan video tape recorder having first and second reproducing heads, means are customarily provided for controlling the across track positions of the first and second reproducing heads into alignment with parallel tracks previously recorded on a video tape.
During still reproduction of interlaced fields, it is desired that both reproducing heads scan the same one of the parallel recorded tracks in order that both reproduced interlaced fields originate in the same frame. If one of the reproducing heads scans one track and the other reproducing head scans a different track, (a phenomenon known as frame reproduction or pairing) two pictures are displayed in the same frame which may have a time difference alternating with every field and hence rendering the object indistinct. Furthermore, if the track scanned by one of the magnetic heads is from one scene and the track scanned by the other magnetic head is from a different scene, the interlaced display of the two completely different video pictures superimposed on each other makes it difficult or even impossible to recognize either of the pictures being displayed. During slow motion, such frame reproduction or pairing may invert the sequence of reproduced fields to blur the displayed picture.
Frame reproduction or pairing, as described in the preceding, comes about because, when a first of the two magnetic heads begins to scan the recorded medium, there is a probability that it will begin scanning at a position on the recording medium equidistant from a pair of adjacent recorded tracks. Although a control system is conventionally employed to coincide a magnetic head with a recorded track, in the special case of equidistant location of the magnetic head from two adjacent tracks, only probability determines in which direction the magnetic head will be deflected and thereby determines which one of the two adjacent tracks will be scanned by the first magnetic head. When the second magnetic head arrives in a location midway between the two recorded tracks, the track to which it will be deflected is also governed by probability. Consequently, there exists a probability that one of the heads will be controlled to coincide with one track and the other head will be controlled to coincide with an adjacent track, thus producing frame reproduction or pairing.
The probability of frame reproduction or pairing is increased by hysteresis in the control elements which are conventionally used to control the across track positions of the magnetic heads. These control elements are suitably bimorph plates which carry head chips at their outer ends and are deflectable by control signals applied thereto. A bimorph plate, when controlled by a control signal to deflect from a neutral or home position, does not return to the neutral or home position when the control signal is removed, but instead remains slightly bent or set in the deflection direction. It is possible that, upon turning off a video tape recorder, one of the bimorph plates may be set in one direction with respect to its neutral or home position and the other bimorph plate may be set in the opposite direction. Upon turning on the video tape recorder, and beginning to scan parallel recorded tracks, the chance is increased of one head being controlled to follow a different recorded track than that followed by the second head.